This invention relates to new and useful improvements in the operation long distance high voltage alternating current power transmission.
The dominant parameters of an electric transmission line are, firstly, shunt capacitance distributed along the conductor to the ground and other nearby conductors and secondly, distributed series self and mutual inductance along the conductor. Consequently, when an electric transmission line is energized with AC voltage, electric current flows out from the conductor through the distributed shunt capacitance paths. In addition, when alternating current flows down a transmission line, the series inductance requires that reactive power (i.e. electric power in which current and voltage are ninety electrical degress out of phase so that no real power exists) must be generated to supply the reactive power loss in the series inductance. The reactive power loss due to the series inductance means that the AC voltage magnitude and phase along the line changes its profile. Furthermore, the current loss in the shunt capacitance results in a change in current being transmitted along the transmission line.
The changes in voltage and current on a transmission line can become extreme for very long distance transmission and can limit the amount of electric power that can be transmitted. In order that maximum electric power transmission can occur, AC voltage and current must remain closely in phase with each other and maintain approximately the same magnitudes along the transmission line.
To maintain voltages and currents near their desired magnitudes and phase relationship, various compensating devices are necessary. The present methods of compensation of AC transmission lines are as follows:
1. Shunt reactive compensation. At each end of the transmission line and sometimes at intermediate positions along the line, inductive reactors, and possibly capacitors also, are arranged for switching in shunt onto the line. When desired power transmission is small or at light load, the transmission line inherent distributed capacitance has a dominant effect which must be compensated by the shunt connected inductive reactors. When the power being transmitted is large, the inherent series inductance of the line is dominant and switching capacitors in shunt onto the line (with the shunt inductors switched off the line) will tend to compensate its effect. Thus, shunt compensation is one form of compensation essential for the steady state operation of long distance high voltage AC power transmission lines.
2. Series reactive compensation. An effective method of counteracting the inherent series inductance of an AC transmission line is to connect series capacitors into the line.
However, shunt and series compensation for AC transmission lines suffer from several inherent problems. Generally, the capacitors and inductors used for shunt compensation are connected to the transmission line by switches or circuit breakers. As the power transmitted is changed to a different value, the inductors or capacitors are switched on or off as required. This operation is satisfactory when steady power is being transmitted or, when power is changed slowly to a different level under the control of the power system operators. However uncontrolled power disturbances occur on a transmission line and may be of sufficient magnitude to cause synchronism to be lost between the ends of the transmission line resulting in disconnection of the line and total loss of power transmission.
Power transmission ceases until resynchronism is achieved and the transmission line is reconnected through its circuit breakers. Generally, shunt connected reactors and capacitors are unable to provide adequate compensation during uncontrolled power disturbances because of the inability to switch them fast enough in a controlled manner. Fast switching of capacitors and inductors is not generally practiced because firstly, high voltage circuit breakers or switches are economically not readily available for fast controlled repetitive operation. Secondly, fast repetitive switching of shunt capacitors and inductors will produce unwanted switching surges on the transmission line.
The main disadvantage of series compensation using series capacitors is the inability of the series capacitors to withstand overcurrents and power surges through the transmission line such as can occur with transmission line faults and uncontrolled power disturbances. Series capacitors are protected under these conditions by applying a short circuit across them. This action, unfortunately, results in reduced ability to maintain synchronism across the length of the transmission line. Further methods for compensating AC transmission lines are:
3. Saturated reactors. Iron cored reactors are subject to saturation, a property which can be used to advantage for shunt compensation. If a saturated reactor is connected in shunt to an AC transmission line, then it will automatically provide the necessary reactive power to compensate the line. It is essential that the reactor operate in saturation for any useful compensation. This forced saturation is achieved with capacitors also connected in shunt, or with the provision to be switched in if the reactor ceases to be saturated. The main advantage of the saturating property of shunt connected iron cored reactors is that the saturation forces the voltage of the AC transmission line (at the reactor terminals) to remain constant at the saturation voltage. Consequently, the desired phase and magnitude balance of current and voltage on the transmission line is preserved for maximum power transfer capability. Thus, saturated reactors with their associated capacitors can be distributed in shunt along the transmission line in order to preserve the necessary voltage and current profile along the line.
Another advantage of saturated reactor compensation is that the desired level of compensation almost instantaneously adjusts itself when changes in power transfer occur. Thus, the line is properly compensated during uncontrolled power disturbances which increases its ability to maintain synchronism between each end. A third advantage of saturated reactor compensation and shunt compensation in general is that short circuits and large current surges can be easily withstood, whereas series capacitor compensation mentioned previously is very vulnerable to these fault conditions.
Disadvantages of saturated reactor compensation are firstly, the saturating property of iron cored reactors creates current harmonics which are undesirable to AC transmission. These current harmonics can be controlled in two ways. One is to divert the harmonics with filters before they enter the transmission line, and the other way is with judicious winding on and construction of the iron core, so that circulating paths for the undesirable lower order harmonic fluxes can be provided.
Both methods for dealing with harmonics in saturated reactors increase its total cost. Secondly, the reactor must operate in the saturation mode which means that shunt capacitance may also be required when the transmission line is transmitting large power. The volt-amp rating of the saturated reactor must be large enough to compensate suitably the transmission line between no load operation and full load operation, as well as provide suitable extra compensation during power surges above full load. The combined rating of the associated shunt capacitors and saturated reactor will be quite large and expensive for most transmission line applications.
It should be mentioned that the saturated reactor rating can be reduced if provision is made to switch shunt capacitors and inductors in and out to just keep the reactor in its saturation mode. For example, at the operation of the transmission line at light load, the capacitors will be switched out so that the saturated reactor will operate as an inductor only. At full load transmission, all or most of the shunt capacitors would be switched in so that the effect of the saturated reactor plus capacitors would be capacitive; the saturated reactor still operates to maintain the transmission line volts at the saturated volts, but capacitors may be necessary to force the reactor into saturation and to supply the capacitive compensating reactive power needed for full power operation.
4. Thyristor controlled reactors and capacitors. It has been shown that dynamic shunt compensation of a transmission line requires variable inductive and capacitive devices. Two methods discussed so far have used firstly switches and secondly saturation property of iron cored reactors as a means of varying inductance and capacitance. Controlled rectifying devices such as silicon controlled rectifiers or thyristors can be used as switches to regulate the current through transformer taps, inductors and capacitors also. Thyristor regulation devices for AC voltage control are used in two ways: Firstly, to act as switches in either the "on" or "off" mode allowing all or none of the AC voltage cycle to pass to the inductors or capacitors. Secondly, to control the firing to the "on" state at each cycle or half cycle so that only a controlled portion of the AC voltage cycle passes to the inductors or capacitors.
Both methods of thyristor switching are effective means of controlling reactive power quickly. Disadvantages to this method of compensation control are:
1. Thyristor switching devices are expensive. PA1 2. Filters would be necessary when thyristor firing is controlled to allow only a portion of the AC voltage cycle to pass. PA1 1. It is normally quite expensive. PA1 2. It is subject to loss of synchronism under violent power disturbances. PA1 3. It adds fault current during short circuits.
5. Synchronous compensation. Another method for transmission line compensation is to use synchronous condensors which are alternating current rotating machines. Synchronous condensors normally generate reactive power in the inductive or capacitive mode which is controlled by adjustment of the machine's field current. Although a most useful transmission line compensating device, the synchronous condensor is restricted by the following disadvantages:
It will therefore be seen that the aforementioned systems suffer from disadvantages and this invention relates to a reactive controller to provide both steady state and dynamic compensation to AC transmission lines at an economically attractive cost.